Degenerative plate circuit detector network



July 25, 1939. P. o. FARNHAM DEGENERATIVE PLATE CIRCUIT DETECTOR NETWORK Filed March l, 1935 www@ i. il

INVENTOR. PAUL O. FARN HAM ATTORNEY.

Patented July 25, 1939 PATENT OFFICE DEGENERATIVE PLATE CIRCUIT DE'IC'lOYR, NETWORK Paul O. Farnham, Boonton, IN. J., assigner Vtp Radio Corporation of America, a corporation of Delaware Application March 1, 1935,'Serial-No. 8,864

Y, 2 Claims.

'My present invention relates tosignal detector networks, and more particularly `to a degenerative detector ofthe platerectication type.

The ideal type of signal-demcdulator network for a radio receiver, and particularly a receiver operatingin the broadcast range, is o-ne that has a substantiallylinear signal inputsignal output of demodulator has'been utilized t'o approximate linear 'demodulatiom but there are various "disadvantages 'inherent in diode detection circuits. Againjthe detection characteristic of a g'ridleak detector possesses a linear portion, but unfortunately the'latter covers only an extremely weak signal range. 'Demodulators of the plate rectication, or'biased, type have been proposedfor use to secure linear detection, but such a linear characteristic is secured only up to some '50 `t'o"l0% modulation of 'the carrier.

Now, in modern broadcast reception there isa positive need for a demodulation device which will have a substantially linear detection characteristic up to 100% modulation of the carrier. Such a positive needv exists because ofthe present demand in 'broadcast radio reception for radio receivers which will be 'capable of reproducing with a high degree of delity the highly 'eflicient modulation of carrier waves at lbroadcasting transmitter stations. It 'is obvious that 'regardless of the efficiency of such modulation, at the broadcast transmitter, such high delity vtransmission will be wasted unless the broadcast receiver is provided with a device which is capable of demodulating the received carrier wave in such a fashion that the demodulation characteristic is linear up to 100% modulation of the carrier,

The various demodulation devices referred Ato above are not readily adapted to this purpose-and maybe said to be highly unsuitable for high fidelity demodulation. Accordingly, it may be stated that it is one of the main objects of my .present invention to provide a radio -frequency signal demodulator network which possesses a substantially linear detection characteristic up to 100% modulation of the received carrier, and which Will additionally possess considerable advantages over a diode demodulation network. the latter having en understood by those skilledin the (Cl. Z- 277) art up to thepresent time as approximating most nearly the 'aforesaid linear characteristic.

' Another important object vof the 'present invention'is to provide-a signal detector circuit having a Vsubstantially linear characteristic vup to 5 %"modulation of the carrier, and which detector circuit `dilfers structurally Yfrom the Well known'typeof plate rectification detector solely in a Vfar/extremelysimple circuit changes.

Another-object ofthe invention Yis to provide a 'degenerative signaldetector of the plate vrectification type wl'iiclfine's'sentially comprises a tube having, Vin addition tothe usualcathode, "grid and anode, a grid biasing impedance ,connected between cathode Aand lgro-und, the biasing iiinpedl5 ance being'so electrically associated with the input and output V'oircuitscf thedetector that degeneration of audio voltage occurs between the control grid and 'cathode of'the detector'tube'with the result that thedetector Ycircuit has imparted to it a substantially linear demo'dulation characteristic lupto '100% lmodulation of the received carrier.

Still anotherobject of the invention-iste pro-` Vide'a degenerative platecircuit detector of the triode type, Lwherein the cathode circuit vof the detector includes agridbias resistor, the resistor being free'of Vany by-passing yof any audio voltage, and a substantial audio voltage being-developedacross the aforesaid bias resistor.

Still otherobjects of the invention are to -improve generally the 'efficiency of 'detection in radio receivers, and more especially tof-provide a high iide'litydete'ction 's'ystemwhich will not only be simple and economical in construction, but reliable'in operation` and readily lasser'nloled in a radioreceiver of modern construction.

The novel features'which 'Ibelieve to be characteristic of inyinventionare set forth inparticularity inthe appended claims, the 'invention .2' itself;V however, jas to both its organization and method o'foperati'on 'will best be'understood vby reference 'to 'the following ,description taken in connection with the'drawi'n'g in which I have inw dicated `diagramm'atically aicircuit organization L10 whereby my "invention may-tbe carried into effect. In the drawing:

Fig. Y1 schematically shows a radio receiving system embodying `the lpresent invention,

Fig. 2` 4graphically shows the characteristics of Vthe detector network embodying the present invention.

Referring now to the accompanying drawing', Fig. 1 schematicallyshows my present invention embodied in a conventional type of radio receiver. Since the present invention is directed to the electrical characteristics of the signal demodulator of the receiving system, the remaining networks of the receiver have been conventionalized, and a brief description will be given with respect to these remaining networks in order to explain the type of receiver in which the present invention is particularly adapted to function. f

The receiving system may be of the so-called .high fidelity type, which is to be utilized for broadcast reception of modulated carrier waves efficiently modulated up to It is to be understood, however, that the invention is not limited to such high fidelity receivers, but is of general use where it is desired to have a highly ef.- cient detection network.

The receiving system shown in Fig. 1 generally comprises a source of signals I, and this source may be the usual grounded antenna circuit, or it may comprise any of vthe well known types of signal collectors used on automobiles, and it may even comprise a radio frequency distribution line as Yused in hotels and apartment houses. The collected side band modulated carrier Waves are impressed upon the radio frequency amplifier section 2 of the receiver. This section of the receiver may comprise a plurality of cascaded stages of tunable radio frequency amplification; or it may consist of the radio frequency amplifier, first detector and intermediate frequency amplifier of a superheterodyne receiver. In any case there will be impressed upon the resonant input circuit 3fof the demodulator, the amplied modulated carrier waves, at carrier frequency or intermediate frequency. If the receiver is of the tuned radio frequency type, then the numeral designates a continuously variable tuning condenser which is uni-controlled with the variable condensers of the preceding amplifier stages.

VOn the other hand if the receiver is of the superheterodyne type, then the numeral 4 designates a fixed condenser which tunes the resonant circuit 3 to the operating intermediate frequency. The detector network demodulates .the modulated `carrier waves, and in the output circuit thereof there is produced the audio voltage ywhich is impresed upon the succeeding audio frequency transmission network. The latter network may comprise one, or more, stages of audio frequency amplification, and the audio output of the last audio amplification stage may beimpressed upon any desired type of reproducer. As stated heretofore, the present invention is particularly adapted for high delity broadcast receivers, and for this reason it will be understood that the various units of the receiver may be designed in such manner that they are best suited for high fidelity broadcast reception. Such design is Well known to those skilled in the art at the present time, and it is unnecessary to explain the electrical construction of the various networks of the system other than the detector network, in this specification.

Considering, then, the detector network it will be Vseen that it comprises an electron discharge tube 5 of the triode type. Of course, the -invention4 isV not limited to tubes of the triode type since any other well known type of ,tube may be used; for example, a shielded tetrode or pentode may be used. The tuned inputr circuit 3 of the detector is connected between the signal input grid 6 and the cathode 1, the low alternating potentials'ide of circuit 3 being connected tothe' cathode .7 .through a condenser. Cg. The operat- `rent from flowing through Re.

ing direct current bias for the grid 6 is obtained by disposing in the cathode lead of the tube the resistor Rc, rthe low alternating potential side of circuit 3 being connected to the negative side of the grid bias resistor through a path which includes resistor Rg.

The anode 8 of the detector tube is connected to a suitable source of positive potential EB through a path which includes the plate circuit load resistor RB. An audio by-pass condenser 9 is connected in series with the plate circuit load resistor, and the audio voltage Ea is derived from across the plate circuit load resistor RB and con.- denser 9. The carrier input applied to the input electrodes of the detector tube 5 is denoted in Fig. 1 by the symbol E0; the grid bias voltage developed across resistor Re is denoted by the vsymbol Ec, and the detector plate current is denoted by the symbol Ip. A radio frequency bypass condenser Cp is connected'between the cathode side of bias resistor Rc and the anode 8 of the detector tube.

Superficially the circuit arrangement of the detector network in Fig. l would seem to resemble to a great extent the familiar plate circuit detector which operates on the lower bend of the plate current-grid voltage characteristic. The

bias voltage developed by the flow of plate current Y tube which would occur if substantial audio voltage were permitted to be developed acrossj the bias resistor Re. In such a conventional type of plate circuit detector, the variation of output audio voltage Ea with percentage modulation on the input carrier voltage E0 is linear only up to some 50 or '70% modulation. In other words, it is entirely unsuitable for the objects and purposes of the present invention wherein it is desired to greatly extend the linear detection characteristic with respect to higherpercentage f modulation on the received carrier.

According to the present invention, however, and as shown in the detector network of Fig. l, the usual shunt audio by-pass condenser connected across the cathode bias resistor is entirely eliminated. The network Rg-Cg prevents carrier degeneration if Rg is large with respect to l/wCg for the carrier frequency; since any carrier frequency output voltage due to plate current of carrier frequency flowing through Re will then produce a negligibly smallr voltage across Cg which is in the control-grid cathode circuit. Of course, in Fig. 1, the presence of Cp prevents most of the carrier frequency plate cur- There isa degeneration of carrier frequency not affected by the Rg-Cg network due to the presence of a plate to controlv grid capacity in the triode of Fig. l in combination with the capacitive plat-e load, Cp, for carrier frequency, and for this reason Cp should be large compared with the plate to grid capacity of the detector, but yet small enough to insure that most of the audio' plate current The usual flows through RB and Re. A good practical an- 75 swer forV this design is to. use a shielded tetrode, or pentode, detector tube.

As a result of this extremely simple change in the conventional biased detector circuit a certain degree Vof sensitivity of the detector is sacriced dueto, the audio degeneration caused by the audio voltage developed across the bias resistor Rc, but on the other hand there is provided a detector device ythat hasa substantially linear detection characteristic up to 100% modulation on the received carrier. In this respect the degenerative biased detector of the present invention will greatly resemble a diode detector operated in an efficient manner, but the present detector network possesses the following advantages over a diode detector:

1. No lossing of carrier input tuned circuit.

2. Less critical circuit conditions than those which must obtain in a diode detector to prevent distortion at high percentage modulation. It is pointed out, in connection with this advantage, that the audio output impedance of a diode detector must be kept substantially equal to the direct current resistance of the leak which is employed.

3. The sensitivity of the present detector network is somewhat higher than that of a diode detector.

From an electrical viewpoint the audio voltage developed across Re when impressed between input electrodes 6 and 1 is of such phase as to produce across the output resistor RB an audio voltage of opposite (or degenerative) phase to the voltage normally produced across RB by the detection process. In Fig. 2 there is furnished a graphic analysis of the characteristics of a degenerative plate circuit detector according to the present invention. There is shown in Fig. 2 a family of curves secured by plotting E as abscissae against Ip as ordinates. The family of curves is secured by using different values of bias voltage Ec.

If the bias resistor Re is by-passed for modulation frequencies, as in the conventional and well known type of plate circuit detector, operation of the detector circuit takes place along the solid line curves such as A-A' for both unmodulated carrier Waves and for modulated carrier waves. It should be particularly noted that by eliminating the by-passing of audio voltages around the bias resistor Re, the modulated carrier input may rise to higher values without drawing grid current than it could if the bias resistor Re were by-passed.

This is explained by the fact that at that part of the modulation cycle at which the carrier input amplitude is a maximum, the instantaneous bias developed across the bias resistor Rc (D. C. plus audio) is impressing a maximum negative bias on the control grid. Hence this constitutes an additional advantage, since it is possible to handle modulated carrier inputs of relatively high amplitude without drawing grid current, and yet demodulate the modulated carrier wave in a substantially linear fashion up to 100% modulation. Such operation cannot be secured With the conventional type of biased, or plate circuit, detector.

The general arrangement of Fig. 1 is capable of yielding greater linearity of audio output up to 100% modulation working along curve C over the modulating cycle than the previous arrangementsl working over such curves as A-A. This may be seen by inspection of Fig. 2, remembering that 100% modulation means that the value of carrier amplitude goes to zero during the modulating cycle. The operating path of curve C is more linear andless` sensitive than that of `A-.A because the audio voltage impressed on the input electrodes is oi such phase4 as to subtract from the audiooutput voltage.v D ue to the saturation effects in the plate current curvesof type A-A and duetov ,the curvature of the bottom of these curves, it is clear Vthat for high Vpercentages` of modulation approaching 100% they willgive less linear operation than curve C. Consider operation with an unmodulated or average carrier amplitude of E0=10 volts. Working on A-A' there is experienced non-linearity and severe distortion for modulation percentages in excess of about 60%, corresponding to a maximum carrier amplitude over the modulating cycle of E0=16 volts; whereas operation along C will give linearity up to 100% modulation, maximum carrier amplitude of 20 volts and minimum carrier arnplitude of zero.

In general, the higher values of Re will permit higher carrier levels on the detector grid before overload is reached due to grid current. In Fig. 1 the values of Cg-Rg should be such that substantially all the audio voltage developed by plate current through Re is impressed upon the input electrodes 6, 1. For an intermediate frequency of 200 kc. impressed at E0 typical constants given merely by way of illustration, for the circuit of Fig. 1 are as follows:

RB=50,000 ohms Rc=50,000 ohms Rg=100,000 ohms 09:0.1 microfarad Cp=500 micromicrofarads It should be mentioned that the audio output E2. may be obtained wholly across Re, in which case RB would be made equal to zero; or, by making RB=R a push-pull audio amplier may be fed, one grid being connected (for audio) to the cathode 7), the other grid (for audio) to the plate (8), while the cathodes of the push-pull stage are at the same audio potentials as the end of Rc which is not connected to (7).

While I have indicated and described a system for carrying my invention into eiect, it Will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A radio frequency signal demodulator network which possesses a substantially linear detection characteristic up to 100 per cent modulation of the receiver carrier, comprising a vacuum tube having cathode, grid and plate electrodesf a tunable input circuit having its high potential terminal connected to the grid and its low potential terminal connected to the cathode, said last mentioned connection including a condenser having a low impedance to carrier frequencies, a grid biasing resistor. having one end connected to the common terminal between one side of said condenser and the cathode, and a second resistor having one end connected to the common terminal between the other side of said condenser and the tuned circuit, the other ends of said resistors being connected to the low potential end of the network, the values of said resistors being such thatdegeneration of audio currents developed across the biasing resistor lo Vto a negativepotential sufficient to prevent the new .of gridV 'current daring. normaiopefatm said biasing kmeans also vproviding a purelypre-g y sistive loadz which .is selectvelycoupled by cnductive coupling atv signal frequencies both v.to tne'grciv circuit and to the anodecircuitinsuch .relation to each that a change of potential across the load'has compensatory effects upon said two circuits. y f u .Y t Y PAUL O. FARNHAM. 

